A scion of an urban patrician family, Heinsius practiced law in Delft until 1679, when he became pensionary (chief executive) of the city. Although he was at first critical of the stadtholder William III's militant opposition to Louis XIV, king of France, he became convinced in the early 1680s of the necessity of opposing Louis's expansionist policies; and in 1689, at William's insistence, he accepted the office of councillor pensionary of Holland. He acted as William's leading Dutch political ally in the War of the Grand Alliance against France, mediating with the States General (national assembly) and serving as one of two Dutch negotiators at the peace settlement at Rijswijk (1697).

After William's death in 1702, Heinsius was unable to maintain control of the States General, which criticized his leadership during the War of the Spanish Succession (Spanish Succession, War of the). His critics believed that while the Dutch had borne a greater share of the war effort, their English allies won greater benefits in the peace Treaty of Utrecht (1713). Heinsius' concentration on foreign affairs had left public finances in poor condition, and after 1713 he sought to avoid further international commitments.